1. Field of the Invention
This invention relates to an alternating current capacitance measuring bridge of high accuracy, capable of measuring variations in capacitance of the order 10.sup.-5 to 10.sup.-6 picofarads.
2. Description of the Prior Art
The problem of measuring small capacitances appears in numerous techniques. For example, in the detection devices of accelerometers of concentric spherical cage and ball type described in U.S. Pat. No. 3,438,267 of the 28th Mar. 1966 to Pierre CONTENSOU, Michel DELATTRE and Michel GAY, the small capacitance variations of a micro-accelerometer are measured. The micro-accelerometer comprises a capacitor formed of a spherical ball concentric to a cage having a diameter slightly greater than that of the ball so that the latter can move in the cage. For example, it is known to measure the displacements or the variations of pressure by means of sensors or capacitive measuring devices in which a variation of distance or a variation of pressure is converted into a variation of capacitance.
The alternating current measuring bridges of known type generally comprise an alternating voltage source feeding the primary winding of a differential transformer, two capacitors, one of which is for reference and preferably adjustable, and the other of which provides the capacitance to be measured, inserted in the circuits of two secondary windings of the differential transformer which have a common terminal and a measuring apparatus for the unbalanced current between the secondary windings inserted in the common terminal.
As in practically all applications, the capacitors whose capacitances are to be measured, have one electrode connected to earth, the impedances relative to earth of the secondary winding of the differential transformer of the measuring bridge are in parallel with the capacitances to be measured which limits the performance and the long-term stability of the said measuring bridge.
It has also been proposed in the prior art to screen the primary winding of the differential transformer by a first screen and the secondary windings of this transformer by a second screen, the second screen also comprising the external envelopes of conductors of the coaxial connections from the secondary windings to the capacitances and being connected to the common terminal of the secondary windings. The mutual impedance of these two screens and the Foucault current developed in the second screen by the magnetic flux coming from the first screen provide a limitation on the performance of the measuring bridges.